1. Field of the Invention
The present invention generally relates to nickel-base alloys. More particularly, this invention relates to castable and weldable nickelalloys exhibiting desirable properties suitable for gas turbine engine applications.
2. Description of the Related Art
The superalloy GTD-222 (U.S. Pat. No. 4,810,467) has a number of desirable properties for gas turbine engine applications, such as nozzles (vanes) in the latter (second and third) stages of the turbine section. The nominal composition of GTD-222 is, by weight, about 19% cobalt, about 22.5% chromium, about 2% tungsten, about 1.2% aluminum, about 2.3% titanium, Al+Ti of about 3.5%, about 0.8% columbium (niobium), about 1.0% tantalum, about 0.01% boron, about 0.01% zirconium, about 0.1% carbon, with the balance essentially nickel and incidental impurities. As with the formulation of other nickelalloys, the development of GTD-222 involved careful and controlled adjustments of the concentrations of certain critical alloying elements to achieve a desired mix of properties. For use in turbine nozzle applications, such properties include high temperature strength, castability, weldability, and resistant to low cycle fatigue, corrosion and oxidation. Unfortunately, when attempting to optimize any one of these desired properties, other properties are often adversely affected. A particular example is weldability and creep resistance, both of which are of great importance for gas turbine engine nozzles. However, greater creep resistance results in an alloy that is more difficult to weld, which is necessary to allow for repairs by welding.
A desirable combination of creep strength and weldability exhibited by GTD-222 is believed to be the result of the use of judicious levels of aluminum, titanium, tantalum and columbium in the alloy. Each of these elements participates in the gamma prime (γ′) precipitation-strengthening phase (Ni3(Ti,Al)). Aluminum and titanium are the key elements in the formation of the gamma-prime phase, while the primary role of tantalum and columbium is to participate in the MC carbide phase. Tantalum and columbium remaining after MC carbide formation plays a lesser but not insignificant role in the formation of the gamma-prime phase.
As noted above, GTD-222 is well suited for use in latter stage gas turbine engine nozzle applications. However, the thermal environment of second stage nozzles can be sufficiently severe to require an oxidation-resistant coating, a thermal barrier coating (TBC), and/or internal cooling. The properties of GTD-222 are sufficient to allow third stage nozzles to achieve the required design life without such additional measures. However, the third stage nozzles of some engine applications do not require the strength of GTD-222. In view of these differences, it would be desirable if alloys were available that were more closely matched to the properties required for second and third stage nozzles of gas turbine engines.